System and method for transmitting data packets in a computer system having a memory hub architecture

ABSTRACT

A system and method for transmitting data packets from a memory hub to a memory controller is disclosed. The system includes an upstream reception port coupled to an upstream link. The upstream reception port receives the data packets from downstream memory hubs. The system further includes a bypass bus coupled to the upstream reception port. The bypass bus transports the data packets from the upstream reception port. The system further includes a temporary storage coupled to the upstream reception port and configured to receive the data packets from the upstream reception port. The system further includes a bypass multiplexer for selectively coupling an upstream transmission port to either one of a core logic circuit, the temporary storage, or the bypass bus. The system further includes a breakpoint logic circuit coupled to the bypass multiplexer and configured to switch the bypass multiplexer to selectively connect the upstream transmission port to either one of the core logic circuit, the bypass bus, or the temporary storage. The system further includes a local memory coupled to the core logic circuit and operable to receive and send the data packets to the core logic circuit.

TECHNICAL FIELD

This invention relates to computer systems, and, more particularly, to asystem and method for transmitting data packets in a computer systemhaving a memory hub architecture.

BACKGROUND OF THE INVENTION

Computer systems use memory devices, such as dynamic random accessmemory (“DRAM”) devices, to store data that are accessed by a processor.These memory devices are normally used as system memory in a computersystem. In a typical computer system, the processor communicates withthe system memory through a processor bus and a memory controller. Theprocessor issues a memory request, which includes a memory command, suchas a read command, and an address designating the location from whichdata or instructions are to be read. The memory controller uses thecommand and address to generate appropriate command signals as well asrow and column addresses, which are applied to the system memory. Inresponse to the commands and addresses, data are transferred between thesystem memory and the processor.

Although the operating speed of memory devices has continuouslyincreased, this increase in operating speed has not kept pace withincreases in the operating speed of processors. Even slower has been theincrease in operating speed of memory controllers coupling processors tomemory devices. The relatively slow speed of memory controllers andmemory devices limits the data bandwidth between the processor and thememory devices.

In addition to the limited bandwidth between processors and memorydevices, the performance of computer systems is also limited by latencyproblems that increase the time required to read data from system memorydevices. More specifically, when a memory device read command is coupledto a system memory device, such as a synchronous DRAM (“SDRAM”) device,the read data are output from the SDRAM device only after a delay ofseveral clock periods. Therefore, although SDRAM devices cansynchronously output burst data at a high data rate, the delay ininitially providing the data can significantly slow the operating speedof a computer system using such SDRAM devices.

One approach to alleviating the memory latency problem is to usemultiple memory devices coupled to the processor through a memory hub.In a memory hub architecture, a system controller or memory controlleris coupled over a high speed link to several memory modules. Typically,the memory modules are coupled in a point-to-point or daisy chainarchitecture such that the memory modules are connected one to anotherin series. Thus, the memory controller is coupled to a first memorymodule over a first high speed link, with the first memory moduleconnected to a second memory module through a second high speed link,and the second memory module coupled to a third memory module through athird high speed link, and so on in a daisy chain fashion.

Each memory module includes a memory hub that is coupled to thecorresponding high speed links and a number of memory devices on themodule, with the memory hubs efficiently routing memory requests andmemory responses between the controller and the memory devices over thehigh speed links. Computer systems employing this architecture can havea higher bandwidth because a processor can access one memory devicewhile another memory device is responding to a prior memory access. Forexample, the processor can output write data to one of the memorydevices in the system while another memory device in the system ispreparing to provide read data to the processor. Moreover, thisarchitecture also provides for easy expansion of the system memorywithout concern for degradation in signal quality as more memory modulesare added, such as occurs in conventional multi drop bus architectures.

FIG. 1 is a block diagram of a system memory 102 that includes memorymodules 104 a and 104 b. The memory module 104 a is coupled to a systemcontroller 108 through a downstream link 128 and an upstream link 136.Each of the memory modules 104 a, 104 b includes a memory hub 112, whichincludes a link interface 116. In the memory module 104 a, the linkinterface 116 is connected to the system controller 108 by the links128, 136. The link interface 116 includes a downstream reception port124 that receives downstream memory requests from the system controller108 over the downstream link 128, and includes an upstream transmissionport 132 that provides upstream memory responses to the systemcontroller over the upstream link 136

The system controller 108 includes a downstream transmission port 140coupled to the downstream link 128 to provide memory requests to thememory module 104 a, and also includes an upstream reception port 144coupled to the upstream link 136 to receive memory responses from thememory module 104 a. The ports 124, 132, 140, 144 and other ports to bediscussed below are designated “physical” interfaces or ports sincethese ports are in what is commonly termed the “physical layer” of acommunications system. In this case, the physical layer corresponds tocomponents providing the actual physical connection and communicationsbetween the system controller 108 and system memory 102 as will beunderstood by those skilled in the art.

The nature of the reception ports 124, 144 and transmission ports 132,140 will depend upon the characteristics of the links 128, 136. Forexample, in the event the links 128, 136 are implemented using opticalcommunications paths, the reception ports 124, 144 will convert opticalsignals received through the optical communications path into electricalsignals and the transmission ports 140, 132 will convert electricalsignals into optical signals that are then transmitted over thecorresponding optical communications path.

In operation, the reception port 124 captures the downstream memoryrequests and provides the captured memory request to local hub circuitry148, which includes control logic for processing the request andaccessing the memory devices 156 over a bus system 152 to provide thecorresponding data when the request packet is directed to the memorymodule 104 a. The reception port 124 also provides the captureddownstream memory request to a downstream transmission port 160 on abypass bus 180. The downstream transmission port 160, in turn, providesthe memory request over the corresponding downstream link 128 to adownstream reception port 124 in the adjacent downstream memory module104 b. The port 124 in module 104 b operates in the same way as thecorresponding port in the module 104 a, namely to capture the memoryrequest and provide the request to the local hub circuitry 148 forprocessing and to provide the request to a downstream transmission port160. The port 160 in the module 104 b then operates in the same way asthe corresponding port in module 104 a to provide the memory requestover the corresponding downstream link 128 to the next downstream memorymodule (not shown in FIG. 1).

The memory hub 112 in the module 104 a further includes an upstreamreception port 164 that receives memory responses over the correspondingupstream link 136 from an upstream transmission port 132 in the adjacentmodule 104 b. An upstream transmission port 132, in turn, provides theresponse over the upstream link 136 to the upstream physical receptionport 144 in the system controller 108. Each of the memory modules 112includes a corresponding downstream reception port 124, upstreamtransmission port 132, downstream transmission port 160, and upstreamreception port 164. Moreover, these ports 124, 132, 160, 164 in eachmodule 104 b operate in the same way as just described for thecorresponding ports in the module 104 a.

In addition to the memory responses from the downstream hubs, the localhub circuitry 148 also receives memory responses from a local memory156. The local memory 156 may be a DRAM type memory device or othersuitable memory devices as will be appreciated by those skilled in theart. The local hub circuitry 148 provides the memory responses from thelocal memory 156 to the upstream transmission port 132 for transmissionover the upstream link 136 to the upstream reception port 144 of thecontroller 108. Thus, the local hub circuitry 148 must monitor andcontrol transmission of memory responses to the system controller 108from the downstream memory module 104 b and from the local memory 156.Since the hub circuitry 148 must monitor and control transmission ofmemory responses to the system controller 108 from the downstream memorymodule 104 b and the local memory 156, the hub circuitry 148 mustdetermine the priority of transmission of the memory responses. The hubcircuitry 148 also must efficiently switch the transmission of memoryresponses from one source to another source. The hub circuitry 148 alsomust switch transmission of memory responses from one source to anothersource at an appropriate time.

The system controller 108 can control the timing of the memory responsesinside the memory hubs 112. However, if there are a large number ofmemory hubs 112 coupled to the system controller 108, it becomescomplicated for the system controller 108 to efficiently determine thepriority of transmission of memory responses and to do the scheduling inall the memory hubs 112. Also when the system controller 108 controlsthe scheduling of memory responses inside the memory hubs 112, thebandwidth available for data transmission is reduced.

Accordingly, there is a need for a system and method for efficientlydetermining the priority of transmission of the memory responses insidethe memory hub 112. There is a need for a system and method forefficiently switching transmission of the memory responses from onesource to another source inside the memory hub 112. There is a need fora system and method for efficiently switching transmission of the memoryresponses from one source to another source at an appropriate point.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method fortransmitting data packets from a memory hub to a memory controller. Inone embodiment, the system includes an upstream reception port coupledto an upstream link. The upstream reception port receives the datapackets from downstream memory hubs. The system further includes abypass bus coupled to the upstream reception port. The bypass bustransports the data packets from the upstream reception port. The systemfurther includes a temporary storage coupled to the upstream receptionport and configured to receive the data packets from the upstreamreception port. The system further includes a bypass multiplexer forselectively coupling an upstream transmission port to either one of acore logic circuit, the temporary storage, or the bypass bus. The systemfurther includes a breakpoint logic circuit coupled to the bypassmultiplexer and configured to switch the bypass multiplexer toselectively connect the upstream transmission port to either one of thecore logic circuit, the bypass bus, or the temporary storage. The systemfurther includes a local memory coupled to the core logic circuit andoperable to receive and send the data packets to the core logic circuit.The bypass bus transports data packets from the downstream hubs to theupstream link when the bypass multiplexer is switched to the bypass bus.The upstream temporary storage stores the data packets from thedownstream hubs when the bypass multiplexer is switched to the corelogic circuit. The core logic circuit transmits the data packets fromthe local memory when the bypass bus is switched to the core logiccircuit. The data packets from the temporary storage are transported tothe upstream link when the bypass multiplexer is switched to thetemporary storage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an existing memory hubs system.

FIG. 2 is a block diagram of a memory hub in accordance with oneembodiment of the invention.

FIG. 3 shows a clock signal and upstream data packets in accordance withone embodiment of the invention.

FIG. 4 shows breakpoints in upstream data packets.

FIG. 5 shows a memory hub in accordance with another embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a block diagram of a memory hub 200 in accordance with oneembodiment of the invention. The memory hub 200 includes a core logiccircuit 204 coupled to the local memory 156. The core logic circuit 204is also coupled to the downstream reception port 124 and the downstreamtransmission port 160. The downstream reception port 124 is coupled tothe system controller 108 (not shown in FIG. 2) via the downstream link128.

The downstream reception port 124 receives read and write requests fromthe system controller 108 (not shown in FIG. 2) over the downstream link128. The core logic circuit 204 receives the read and write requestsfrom the downstream reception port 124. The core logic circuit 204 sendsto the local memory 156 those read and write requests that are destinedfor the local memory 156. Read and write requests that are destined fordownstream hubs (not shown in FIG. 2) are moved from the reception port124 to the transmission port 160 on the downstream bypass bus.

The memory hub 200 further includes the upstream transmission port 132that is linked to the system controller 108 by the upstream link 136. Aswill be discussed further, read and write responses from the core logiccircuit 204 and the downstream hubs (not shown in FIG. 2) aretransmitted by the upstream transmission port 132 to the systemcontroller 108 over the upstream link 136. A read response includes readdata from the local memory 156 and a write response indicates one ormore write requests have been completed.

The memory hub 200 further includes a bypass multiplexer 212 coupled tothe core logic 204 and a temporary storage 216. The bypass multiplexer212 is also connected to the upstream reception port 164 via a bypassbus 220. The bypass multiplexer 212 selectively couples either the corelogic 204, the bypass bus 220 or the temporary storage 216 to theupstream transmission port 132.

In operation, read and write responses from the downstream hubs arereceived by the upstream reception port 164 over the upstream link 136and are passed on to the upstream transmission port 132 over the bypassbus 220 and through bypass multiplexer 212. Read responses are receivedby the core logic 204 from the local memory 156 and are passed on to theupstream transmission port 132 through the bypass multiplexer 212. Writeresponses are generated in the core logic 204 and are also passed on tothe upstream transmission port 132 through the bypass multiplexer 212.As will be discussed further, when the bypass multiplexer 212 couplesthe core logic 204 to the upstream transmission port 132, the temporarystorage 216 is used to temporarily store read and write responses fromthe downstream hubs. In the following description, write and readresponses from the core logic 204, the downstream hubs and the temporarystorage 216 will be referred to simply as “data.”

As described above, the upstream transmission port 132 transmits data,over the upstream link 136, originating from one of several sources: (1)the local memory 156; (2) downstream hubs; and the temporary storage216. The multiplexer 212 selectively couples the upstream link 136,through the transmission port 132, to either the core logic 204, thebypass bus 220 or the temporary storage 216. The multiplexer 212 isswitched so that data originating from either the core logic 204, thebypass bus 220 or the temporary storage 216 are transmitted over theupstream link 136 to the system controller 108. A breakpoint logic 208coupled to the bypass multiplexer 212 provides the switching algorithmto the bypass multiplexer 212. The switching algorithm locates switchpoints (also referred to as breakpoints) when a switch may occur. If theswitching algorithm locates a breakpoint and it is determined that aswitch should be made to another data source that has data available,the bypass multiplexer is switched so that the new data source iscoupled to the upstream link 136 through the upstream transmission port132.

In general, data is transferred among the memory hub 200, the systemcontroller 108 and downstream hubs in a fixed data packet format. A datapacket includes a beginning and an end. The breakpoint logic 208determines the beginning or end of a data packet, and a switch is madeat the beginning or end of a data packet.

In one embodiment, the core logic 204 operates at 400 MHz. The receptionports 124, 164, and the transmission ports 132, 160 operate at 1.6 GHz.The upstream link 136 and the downstream link 128 operate at 6.4 GHz.

The operating speed of these devices are selected due to designrequirements. The upstream and downstream links are operated at veryhigh speed (6.4 GHz) in order to provide a large bandwidth. However, thetransmission ports 136, 160, the reception ports 124, 164, and the corelogic 204 cannot be operated at such high speed using currenttechnology. Thus, as data is transferred from the downstream link to thereception port, the transfer speed is reduced. As data is moved to thecore logic, the speed is reduced further.

FIG. 3 shows a clock signal, indicated as a 4X clock, where X=400 MHz,and data packets in accordance with one embodiment of the invention. Thelength of the data packets depends on the type of data beingtransferred. A write response data packet transfers limited amount ofinformation, primarily containing an ID number and control bitsindicating that it is a write response. A read response data packetincludes the same information as the write response data packet, but inaddition the read response data packet includes the read data beingreturned. Thus the response data packet is longer than the writeresponse data packet.

In FIG. 3, the clock being used is a 4X clock which transfers 64 bits (8bytes) in each clock cycle. In the example of FIG. 3, the read responsedata packet includes 64 bytes of data. These 64 bytes take 8 clockcycles to transfer. The read response data packet also includes 4 headerbytes and 4 Cycle Redundancy Code (CRC) bytes, which require 1 clockcycles to transfer. Thus, the read response data packet requires a totalof 9 clock cycles to transfer. The write response includes 32 bytes ofdata (multiple write completes), 4 bytes of header and 4 bytes of CRC.As understood by those skilled in the art, the header bytes are controlbytes, and the CRC bytes are used as standard error checking mechanism.

FIG. 3 also shows an idle packet, which is four clock cycles long. Theidle packet contains 4 header bytes and 28 no operation (NOP) bytes. Theidle packet is sent on the upstream bus by the downstream hubs when thehubs do not have any data to send. The idle packet allows the breakpointlogic to switch when no data is being sent by the downstream hubs.

In one embodiment, a data packet moves from the upstream reception port164 to the upstream transmission port 132 in one 1.6 GHz clock period.However, the breakpoint logic 208, which switches the bypass multiplexer212, requires three clock periods to complete the switch because of thetime required to process a decode and drive logic to switch the bypassmultiplexer 212. Thus, the beginning of the data packet is located as itenters the memory hub 200, and then switching is initiated three clockcycles prior to the breakpoint so that the bypass multiplexer 212 isswitched in time as the data packet arrives.

FIG. 4 shows valid breakpoints in data packets. The bypass multiplexer212 is switched at valid breakpoints. A valid breakpoint exists betweentwo read responses, between a read response and a write response, andbetween a write response and a read response.

As described before, the determination that the bypass multiplexer 212will be switched is made three clock cycles before the arrival of a datapacket. By looking ahead three clock cycles before the data arrives, theswitching process of the bypass multiplexer 212 can begin so that theswitch coincides with the data arrival. The write response data packetin FIG. 4 shows that a determination that the bypass multiplexer 212will be switched is made three clock cycles before a breakpoint.

FIG. 5 shows a memory hub 500 in accordance with another embodiment ofthe invention. The memory hub 500 includes the elements shown in FIG. 2and described before. In addition, the memory hub 500 includes twotemporary storages: an upstream buffer 512, and a bypass FIFO 516coupled to the bypass multiplexer 212 and the bypass bus 220. The bypassFIFO is a high speed buffer operating at 4X clock speed, where X=400MHz. The upstream buffer is a normal speed buffer operating at 1X clockspeed.

When the bypass multiplexer 212 is switched to the core logic 204,incoming data packets from the downstream hubs are first stored in thebypass FIFO 516. Since the bypass FIFO 516 operates at high speed (4Xclock speed), the bypass FIFO 516 can transfer data packets from itsinput to its output very quickly. Thus, if the core logic 204 completessending data packet and the bypass multiplexer switches to the temporarystorages, the data from the bypass FIFO 516 is available immediately.

However, if the bypass multiplexer 212 remains switched to the corelogic 204, incoming data packets from the downstream hubs fill up thebypass FIFO 516. When the bypass FIFO 516 is filled up, the upstreambuffer 512 is used to store data packets. As will be understood by thoseskilled in the art, the bypass FIFO 516 is fast, but is expensive toimplement. Thus a small bypass FIFO 516 is typically used. The upstreambuffer 512 is slower, but is less expensive to implement. Thus, a largeupstream buffer 516 is used.

The memory hub 500 includes clock domain change circuits 520, 524, 508.As noted before, since the downstream ports 124, 160 operate atdifferent clock frequency than the core logic 204, the downstream ports124, 160 are not synchronous with the core logic 204. Thus, data packetscannot be directly transferred between the core logic and the downstreamports 124, 160. The clock domain change circuit 520 allows transfer ofdata packets from the downstream port 124 to the core logic 204, and theclock domain change circuit 524 allow the transfer of data packets fromthe core logic 204 to the downstream port 160. The core logic 204 issynchronous with the bypass multiplexer 212, and the clock domain changecircuit 508 allows the transfer of data packets from the core logic 204to the bypass multiplexer 212 through a core upstream FIFO 504.

In one embodiment, after power up, the breakpoint control logic 208initially switches the bypass multiplexer 212 to the bypass bus 220,thus connecting the bypass bus 220 to the upstream link 136. The bypassbus 220 remains connected to the upstream link 136 until the core logic204 has data to be sent and a breakpoint is available on the bypass bus220. If the core logic 204 has data available and a breakpoint isavailable, the bypass multiplexer 212 is switched to the core logic 212.

When the bypass multiplexer 212 is switched to the bypass bus 220, dataon the bypass bus 220 is sent to upstream link 136. When the bypassmultiplexer 212 is switched to the core logic 204, data from the corelogic 204 is sent to the upstream link 136. While the bypass multiplexer212 remains switched to the core logic 204, incoming data on the bypassbus 220 is sent first to the bypass FIFO 516. When the bypass FIFO 516is filled up, data is next to the upstream buffer 512.

In one embodiment, the bypass multiplexer 212 remains switched to thecore logic 204 until the core logic 204 is empty or if a higher priorityrequires a switch. A higher priority is determined if the temporarystorages, i.e., the bypass FIFO 516 or the upstream buffer 512, haveavailable data. When the bypass multiplexer 212 is switched away fromthe core logic 204, the multiplexer 212 is first switched to the bypassFIFO 516. The data in the bypass FIFO 516 is sent upstream over theupstream link 136 until the bypass FIFO is exhausted. In general, afterthe bypass FIFO 516 is exhausted, the bypass multiplexer 212 is nextswitched to the upstream buffer 512, which is then emptied.

If the core logic 204 has data available, a switch can be made from thebypass FIFO 516 to the core logic 204 even though the bypass FIFO hasnot been exhausted. If a switch is made from the bypass FIFO 516 to thecore logic 204, the next switch is made back to the bypass FIFO 516 inorder to send the upstream data in the order it was received. When thebypass FIFO 516 empties, data is next taken from the upstream buffer512. A switch to the core logic 204 can be made from the upstream buffer512 even though the upstream buffer has not been exhausted. However, thenext switch is made back to the upstream buffer 512 in order to send theupstream data in the order it was received.

After the bypass FIFO 516 and the upstream buffer 512 are cleared, themultiplexer 212 is normally switched to the bypass buss 220. If,however, the core logic 204 has available data, the multiplexer 212 isswitched to the core logic 204. As discussed before, while the bypassmultiplexer 212 is switched to the core logic 204, upstream data isfirst loaded into the bypass FIFO 516 and then into the upstream buffer512. When the bypass multiplexer 212 is switched to the temporarystorages, the bypass FIFO 516 is emptied first and then the upstreambuffer 512 is emptied next. After the bypass FIFO 516 is emptied, it isnot loaded again until the upstream buffer 512 has been emptied.

In the preceding description, certain details were set forth to providea sufficient understanding of the present invention. One skilled in theart will appreciate, however, that the invention may be practicedwithout these particular details. Furthermore, one skilled in the artwill appreciate that the example embodiments described above do notlimit the scope of the present invention, and will also understand thatvarious equivalent embodiments or combinations of the disclosed exampleembodiments are within the scope of the present invention. Illustrativeexamples set forth above are intended only to further illustrate certaindetails of the various embodiments, and should not be interpreted aslimiting the scope of the present invention. Also, in the descriptionabove the operation of well known components has not been shown ordescribed in detail to avoid unnecessarily obscuring the presentinvention. Finally, the invention is to be limited only by the appendedclaims, and is not limited to the described examples or embodiments ofthe invention.

1. In a memory system having a plurality of memory modules each of whichcontains a memory hub connected to a plurality of memory devices, asystem in at least one memory hub for receiving data packets through afirst upstream link from a downstream memory module and the memorydevices connected to the memory hub and for transmitting the receiveddata packets through a second upstream link to either an upstream memorymodule or a controller, the system comprising: an upstream receptionport coupled to the first upstream link and operable to receive datapackets from the downstream memory module; a bypass bus coupled to theupstream reception port and operable to receive the data packets fromthe upstream reception port and to transport the data packets; atemporary storage coupled to the upstream reception port and operable toreceive the data packets from the upstream reception port; an upstreamtransmission port coupled to the second upstream link, the secondupstream link being isolated from the first upstream link; a bypassmultiplexer for selectively coupling the upstream transmission port toeach of a core logic circuit, the temporary storage, and the bypass bus;and a breakpoint logic circuit coupled to the bypass multiplexer andoperable to switch the bypass multiplexer to selectively connect theupstream transmission port to each of the core logic circuit, the bypassbus, and the temporary storage.
 2. The system of claim 1 furthercomprising a local memory coupled to the core logic circuit and operableto receive and send the data packets to the core logic circuit.
 3. Thesystem of claim 1 wherein the upstream transmission port is coupled tothe memory controller by the second upstream link.
 4. The system ofclaim 1 wherein the bypass bus transports data packets from thedownstream memory module to the second upstream link when the bypassmultiplexer is switched to the bypass bus.
 5. The system of claim 1wherein the temporary storage stores the data packets from thedownstream memory module when the bypass multiplexer is switched to thecore logic circuit.
 6. The system of claim 1 wherein the data packetsfrom the temporary storage are transported to the second upstream linkwhen the bypass multiplexer is switched to the temporary storage.
 7. Thesystem of claim 1 wherein the bypass multiplexer is initially switchedto the bypass bus after the memory hub is powered up.
 8. The system ofclaim 1 wherein the bypass multiplexer is switched to the core logiccircuit when the core logic circuit has data packets available.
 9. Thesystem of claim 1 wherein the bypass multiplexer is switched at thebeginning of a data packet.
 10. The system of claim 1 wherein the bypassmultiplexer is switched at the end of a data packet.
 11. A memorysystem, comprising: a memory controller; a plurality of memory hubs eachof which is connected to a plurality of memory devices, the memory hubsbeing coupled to adjacent memory hubs through respective first andsecond upstream links that are isolated from each other, at least one ofthe memory hubs being coupled to the memory controller through therespective second upstream link; a system in at least some of the memoryhubs for transmitting data packets from the memory hub to the secondupstream link comprising: an upstream reception port coupled to thefirst upstream link and operable to receive data packets from thedownstream memory hub though the first upstream link; a bypass buscoupled to the upstream reception port and operable to receive the datapackets from the upstream reception port and to transport the datapackets; a temporary storage coupled to the upstream reception port andoperable to receive the data packets from the upstream reception port;an upstream transmission port coupled to the second upstream link; abypass multiplexer for selectively coupling the upstream transmissionport to each of a core logic circuit, the temporary storage, and thebypass bus; and a breakpoint logic circuit coupled to the bypassmultiplexer and operable to switch the bypass multiplexer to selectivelyconnect the upstream transmission port to each of the core logiccircuit, the bypass bus, and the temporary storage.
 12. The memorysystem of claim 11 wherein the system for transmitting data packets fromthe memory hub to the memory controller further comprises a local memorycoupled to the core logic circuit and operable to receive and send thedata packets to the core logic circuit.
 13. The memory system of claim11 wherein the upstream transmission port is coupled to the memorycontroller by the second upstream link.
 14. The memory system of claim11 wherein the bypass bus transports data packets from the downstreammemory hubs to the second upstream link when the bypass multiplexer isswitched to the bypass bus.
 15. The memory system of claim 11 whereinthe temporary storage stores the data packets from the downstream memoryhub when the bypass multiplexer is switched to the core logic circuit.16. The memory system of claim 11 wherein the core logic circuittransmits the data packets from the local memory when the bypass bus isswitched to the core logic circuit.
 17. The memory system of claim 11wherein the data packets from the temporary storage are transported tothe second upstream link when the bypass multiplexer is switched to thetemporary storage.
 18. The system of claim 11 wherein the bypassmultiplexer is initially switched to the bypass bus after the memory hubis powered up.
 19. The system of claim 11 wherein the bypass multiplexeris switched to the core logic circuit when the core logic circuit hasdata packets available.
 20. The system of claim 11 wherein the bypassmultiplexer is switched at the beginning of a data packet.
 21. Thesystem of claim 11 wherein the bypass multiplexer is switched at the endof a data packet.
 22. A system in a first memory hub for receiving datapackets from a second memory hub, and for transmitting data from thefirst memory hub to a memory controller, comprising: a first upstreamlink coupled to the second memory hub; a second upstream link coupled tothe memory controller, the second upstream link being isolated from thefirst upstream link; an upstream reception port coupled to the firstupstream link and operable to receive data packets from the secondmemory hubs; an upstream transmission port coupled to the secondupstream link and operable to transmit data packets to the memorycontroller; a bypass bus coupled to the upstream reception port andoperable to receive the data packets from the upstream reception portand to transport the data packets; an upstream buffer coupled to theupstream reception port and operable to receive the data packets fromthe upstream reception port; a bypass FIFO coupled to the upstreamreception port and operable to receive the data packets from theupstream reception port; a bypass multiplexer for selectively couplingthe upstream transmission port to each of a core logic circuit, theupstream buffer, the bypass FIFO and the bypass bus; and a breakpointlogic circuit coupled to the bypass multiplexer and operable to switchthe bypass multiplexer to selectively connect the upstream transmissionport to each of the core logic circuit, the bypass bus, the upstreambuffer and the bypass FIFO.
 23. The system of claim 22 furthercomprising a local memory coupled to the core logic circuit and operableto receive and send the data packets to the core logic circuit.
 24. Thesystem of claim 22 wherein the bypass bus transports data packets fromthe downstream hubs to the second upstream link when the bypassmultiplexer is switched to the bypass bus.
 25. The system of claim 22wherein the bypass FIFO stores the data packets from the downstream hubswhen the bypass multiplexer is switched to the core logic circuit. 26.The system of claim 22 wherein the upstream buffer stores the datapackets from the downstream hubs after the bypass FIFO is filled up. 27.The system of claim 22 wherein the bypass multiplexer is initiallyswitched to the bypass bus after the memory hub is powered up.
 28. Thesystem of claim 22 wherein the bypass multiplexer is switched to thecore logic circuit when the core logic circuit has data packetsavailable.
 29. A method for transmitting data packets received by amemory hub through a first upstream link to a memory controller over asecond upstream link that is isolated from the first upstream link, thedata packets originating from a local memory and downstream hubs of thecomputer system, comprising: transmitting a data packet from the localmemory by coupling the local memory to the second upstream link; if adata packet is not being transmitted from the local memory when a datapacket is received from the downstream hub, transmitting the datapackets from the downstream hubs by coupling a bypass bus between thefirst upstream link and the second upstream link; if a data packet isbeing transmitted from the local memory when a data packet is receivedfrom the downstream hub, storing the data packets from the downstreamhubs in a temporary storage while the data packets from the local memoryis being transmitted; and when a data packet is not being transmittedfrom the local memory, transmitting the data packets from the temporarystorage by coupling the temporary storage to the second upstream link.30. The method of claim 29 further comprising transmitting the datapackets from the local memory when there is a data packets from thelocal memory.
 31. The method of claim 29 further comprising transmittingthe data packets from the temporary storage prior to transmitting thedata packets from the downstream hubs.
 32. In a memory module having amemory device providing read data, an upstream reception port throughwhich data are received, and an upstream transmission port through whichdata are transmitted, the upstream transmission port being isolated fromthe upstream reception port, a method of transmitting data from thememory module comprising: transmitting the read data provided by thememory device through the upstream transmission port; receiving datathrough the upstream reception port; if the read data provided by thememory device are not being transmitted through the upstreamtransmission port when the data are received through the upstreamreception port, transferring the data received through the upstreamreception port to the upstream transmission port for transmissionthrough the upstream transmission port; if the read data provided by thememory device are being transmitted through the upstream transmissionport when the data are received through the upstream reception port,temporarily storing the data received through the upstream receptionport; and subsequently transmitting the temporarily stored data throughthe upstream transmission port.
 33. The method of claim 32 wherein theact of subsequently transmitting the temporarily stored data through theupstream transmission port comprises transmitting the temporarily storeddata through the upstream transmission port when data provided by thememory device are not being transmitted through the upstreamtransmission port.
 34. The method of claim 32 wherein the act ofsubsequently transmitting the temporarily stored data through theupstream transmission port comprises transmitting the temporarily storeddata through the upstream transmission port when data transferred fromthe upstream reception port to the upstream transmission port are notbeing transmitted through the upstream transmission port.
 35. The methodof claim 32 wherein the act of transferring the data received throughthe upstream reception port to the upstream transmission port fortransmission through the upstream transmission port comprisestransferring the data received through the upstream reception port tothe upstream transmission port for transmission through the upstreamtransmission port only if the temporarily stored data are not beingtransmitted through the upstream transmission port.